Optical fiber connectors are used to couple optically one optical fiber to another optical fiber, or optical fibers to optical devices such as LEDs, lasers or detectors. Such connectors form an essential part of substantially any optical fiber communications systems, and the industry has expended a substantial effort on their development.
An optical fiber used for communications includes a core and a cladding disposed thereabout. Considering the fact that the optical fiber may have an outer diameter of 125 microns over the cladding, the connection of two optical fibers such that their cores in the range of about 8 to 62.5 microns are aligned is a formidable task. Several connectors are available commercially for establishing a connection between optical fibers.
One connector is referred to as a biconic connector. It includes facilities for holding two plugs each of which terminates an optical fiber and each of which has a conically shaped end portion. The optical fiber end terminates in a secondary pedestal which extends beyond a primary pedestal of the plug. Two plugs are received in opposite ends of a sleeve which is mounted in a housing. The sleeve includes opposed, conically shaped cavities for receiving end portions of the plugs and for holding them in a manner to cause the end faces of the optical fibers to touch or to be spaced apart slightly. The plugs and the sleeves, which are molded, are controlled such that their mating surfaces cause the optical fibers to become aligned when the plugs are received in the sleeve.
Another connector for making an optical connection is referred to as an ST.RTM. connector, ST being a registered trademark of AT&T. It includes a coupling having a plug-receiving tubular portion at each end thereof. Each tubular portion is provided with a longitudinally extending slot. A sleeve which floats somewhat within the coupling is adapted to receive coaxially end portions of two plug assemblies each of which includes a plug adapted to terminate an optical fiber. The plug sometimes is referred to as a terminus. Each plug has a passageway extending longitudinally therethrough for receiving an end portion of an optical fiber and is mounted in a plug body having an alignment key projecting radially therefrom. When the plug body is received in a tubular portion of the coupling, the alignment key is received in the slot which extends along the tubular portion. A retaining pin which projects radially from each tubular portion of the coupling is received in a slot of a cap which encloses the associated plug and plug body and which encloses a tubular portion of the coupling. The slot in the cap includes a portion in which the retaining pin of the coupling is received to lock the plug assembly to the coupling. The plug is biased outwardly of the cap by a compression spring disposed about the plug body.
The just-described connector is advantageous in that the plugs are made of a ceramic material and are not molded. As a result, the plugs may be machined with close tolerances which is advantageous when dealing with optical fibers having relatively small dimensions. Further, the passageways in the plugs that are destined to receive the optical fibers are made cleanly without the molding flash which may be expected in other kinds of connectors and which could damage the optical fibers. Of course, the plug may be molded, if desired.
Connectors of this type, also referred to as "ferrule-type" connectors, rely on the alignment of the outside surfaces of the plugs to provide fiber alignment. For this approach to be satisfactory, the fiber-receiving capillary bore of a plug should be concentric with the outer cylindrical surface of the plug. Furthermore, in some embodiments, the optical fiber is flush with the mating end face of the plug, and the two mating end faces in a connector are normal to the fiber axis, to within relatively close tolerances. Substantial deviations from these conditions tend to result in added signal loss.
A problem with the use of the ferrule connector relates to the potential for optical disconnection of optical fiber end faces or of an optical fiber end face and an optical device to which it is connected. It will be recalled that the plug is biased outwardly of the cap by a compression spring. Should sufficient force by applied inadvertently axially to the optical fiber cable which is terminated by the plug, in a direction away from the optical connection, the plug will be moved in a direction outwardly from the center of the sleeve causing effectively a disconnection of the optical fiber end faces or of a fiber end face and a device and hence a disconnection of optical transmission.
Also, because of the construction of the housing, the plug, upon the application of forces to the cable in a direction transversely of the axis of the connector will result in a turning of the plug about a fulcrum located between the center of the sleeve and the end of the cap. This results in a canting of the end face of the plug and angular spacing thereof from the other plug or device thereby causing an optical disconnection or increased transmission loss.
In attempting to provide a solution to the problem of unintended longitudinal and turning movement of the plug, one must be mindful of the problem of compatibility. Because many ST.RTM. connectors already are in use, it would be imprudent to provide plug assemblies which overcome the problem of such unwanted axial or angular movement but which are not compatible with existing plugs and sleeves. A biconic connector which includes provisions for preventing inadvertent disconnection and which is compatible with other biconic connectors already in use is disclosed and claimed in commonly assigned application Ser. No. 068,586 filed of even date herewith in the name of N.R. Lampert.
A still further problem relates to the assembly of the plug assembly with the coupling housing. That assembly involves the turning of the plug assembly with respect to the housing. In many installations such as a central office, for example, the coupling is attached to a panel as one of a mass of such couplings in an array. Because of the closeness of the connector couplings in the array, it becomes somewhat difficult for a craftsman to grasp the plug assembly near its plug end to turn it into the coupling housing. The portion of the cable adjacent to the cable entrance end cannot be grasped and used to turn the plug inasmuch as the cable and plug are capable of rotating relative to the cap.
Seemingly, the prior art has not yet offered a solution to these problems. What is needed is an ST.RTM. connector which is compatible with ST.RTM. connector systems already in use and which includes facilities that prevent losses or inadvertent disconnection caused by bending of the cable at is entrance to the connector. Also desired is the ability to be able to turn the plug assembly for assembly to a coupling housing by turning the cable at a point remote from its entrance to the plug assembly. Further, the sought-after connector should be capable, if desired, of avoiding inadvertent disconnection upon the application of axial forces.